Ultrafast lasers can deliver very short (ultrafast) pulses at relatively high pulse repetition rates. By way of example pulses may have a duration of 10 picoseconds (ps) or less at pulse repetition rates between about 75 Megahertz (MHz) and 1 Gigahertz (GHz) or greater. The pulse duration of a passively modelocked laser is determined, inter alia, by the gain-medium in the resonator and the method of passive modelocking. The pulse repetition rate of a passively modelocked laser is determined by the optical length of the resonator of the laser.
In many applications of ultrafast laser pulses such as time-resolved coherent anti-Stokes Raman spectroscopy (CARS) and CARS imaging studies, diagnosis of the ultrafast pulses, such as determination of pulse amplitude and duration, is required. As the duration of pulses decreases, conventional optical detectors such as photodiode-detectors, become less able to provide a signal that gives an accurate representation of the amplitude and duration of the pulses. Simply stated, this is because the length of time it takes a detector to respond to the pulse is greater than the duration of the pulse.
In order to avoid this problem pulse measurement techniques based on correlation of two pulses have been extensively developed. In these methods what is measured is a signal provided by observing an effect produced in a detecting medium that is produced when the medium receives two pulses simultaneously. The phase of one of the pulses is progressively changed with respect to the other. The effect to be observed (detected) is at a minimum when the pulses only just overlap in time and at a maximum when the pulses exactly overlap, i.e., are exactly in phase. Numerical analysis techniques are used to extract pulse characterization data from signals representing the magnitude of the observed effect as a function of phase.
Correlation techniques may be divided into autocorrelation techniques and cross-correlation techniques. In auto correlation the two pulses to be progressively overlapped are created by optically dividing a single pulse into two components. One of the pulse components is sent along a fixed optical path to the two-pulse detecting medium and the other is sent via another optical path of a variably different length from the length of the fixed optical path. Variation of this path length is used to vary the time between arrival of the pulse components at the detecting medium. In cross-correlation each of the two pulses is supplied by a separate laser. Pulses from one of the lasers travels to the detecting medium via a fixed path. Pulses from the other lasers travel to the detecting medium via a variable path. Methods for extracting pulse characteristics cross-correlation and auto-correlation methods are similar and are well known in the art. A summary providing examples of both techniques in provided in a book “Femtosecond Laser Pulses—Priniciples and Experiments”, C. Rullière (Ed), pp 177–201. A detailed description of cross-correlation is provided in a paper “Ultrafast Diagnostics”, J. C. Diels et al., Rev. Phys. Appl., 12, 1605 (1987).
Common to prior-art autocorrelation and cross correlation techniques alike is a requirement for an optical delay line of variable length. Often these delay lines are required to be relatively long, particularly for picosecond pulses. If such delay lines are used in time resolved spectroscopy studies for example 3 nanosecond (ns) correlation requires a delay line having a length of about one meter (1.0 m). The delay lines must be mechanically, precisely constructed, such that the path length can be varied without significantly varying the physical superposition of temporally superposed individual pulses or pulse components. There is a need for a correlation technique that does not require a variable optical delay line for varying the temporal superposition of pulses and preferably, does not require an optical delay line at all.